Integration of the semiconductor laser and the transistor is a very important step in the realization of opto-electronic integrated circuits. This combination would be a key building block for optical data communications and information processing systems. The monolithic integration of a field effect transistor (FET) with a laser would be very attractive because of the potential for high speed, high density, reliability and low cost (Wada, T. Sakurai, and T. Nakagami, IEEE J. Quantum Electron, QE-22,805, 1986). This integration has been difficult until now because of the inherent differences in growth structure and operation of these two devices (S. R. Forrest, Proc. IEEE, Vol. 75, No. 11, pp 1488-1498). In lasers the optical guiding and confinement layers usually dictate a vertical device thickness of 2-3 microns with current flow in the vertical direction. However in FETS the current flow is confined to a narrow channel in the plane of the layers. Traditionally in III-V FET's the device thicknesses must be small in the vertical direction because the gate electrode must be in close proximity to the channel in order to increase the field effect. Hence, FET structures such as the HEMT or SISFET have metal electrodes within about 300-500 A from the conducting interface. A further difficulty lies in the fabrication technologies since the isolation and passivation technologies for the two are inherently different.
Previously we had introduced a new type of field effect transistor (HFET-Heterostructure Field-Effect Transistor) which utilized inversion at a heterointerface produced by a novel planar doping structure (G. W. Taylor and J. G. Simmons, Electronics Letters, Jul. 17, 1986, Vol. 22, No. 15, pp. 784-786; G. W. Taylor, M. S. Lebby, T. Y. Chang, R. N. Gnail, N. Sauer, B. Tell and J. G. Simmons, Electronics Letters, Jan. 16, 1987, Vol. 23, No. 2, pp. 77-79). It was indicated that this FET was ideal for integration with lasers (G. W. Taylor, D. L. Crawford, P. A. Kiely, S. K. Sargood, P. Cooke, A. Izabelle, T. Y. Chang, B. Tell, M. S. Lebby, K. Brown-Boebeler and J. G. Simmon, IEEE Transactions on Electron Devices, Vol. 35, No. 12, pp. 2466; G. W. Taylor, D. L. crawford, P. A. Kiely, P. Cooke, S. Sargood, A. Izabelle, T. Y. Chang, B. Tell, M. S. Lebby, K. Brown-Goebeler and J. G. Simmons, Proc. SPIE, Vol. 994, pp. 251-257) and other optical devices based on the same concept because it introduced an ohmic contact for the gate on a highly p-doped layer. The gate contact is separated by at least 1 micron of neutral material from the active channel (the gate barrier is provided by the built-in depletion region) in contrast to the HEMT, the MESFET and the SISFET where the gate barrier is a schottky close to the interface. The larger gate barrier in the HFET provides for enhanced supply voltages and circuit noise margins. It also was fabricated with a substantial p doping and wide bandgap material below the heterointerface to provide the formation of a natural graded index structure. In the versions of the HFET that have been reported it was fabricated with a self-aligned refractory technology which is ideally suited for a high yield and high density technology (G. W. Taylor, P. A. Kiely, A. Izabelle, D. L. Crawford, M. S. Lebby, T. Y. Chang, B. Tell, K. Brown-Goebeler and J. G. Simmons, IEEE Electron Device Letters, Vol. 10, No. 2, February, 1989; R. S. Mand, S. Eicher and A. J. Springthorpe, Electronics Letter, Vol. 25, March, 1989, pp. 386-387).